1. Field of the Invention
This invention relates to moisture-curable ethylene copolymer compositions useful as hot-melt adhesives, and more particularly it relates to such compositions containing hydrolysable silane groups. The moisture-curable copolymers are prepared from defined ethylene copolymers containing a specified level of carboxylic functionality, by reaction with epoxy-silanes.
2. Description of Related Art
Use of adhesives applied as a hot melt is growing rapidly because hot melts allow fast production speeds, and solvents are unnecessary. Use of solvents is increasingly regarded as undesirable. However, for many applications limited temperature resistance of hot-melt adhesives is a major deficiency. Often, a `soft` adhesive which maintains an adhesive bond well at ambient temperatures becomes weak at higher temperatures, and is not suitable for use at such temperatures.
Ethylene copolymers are well known for use as inexpensive hot-melt adhesives. Comonomers such as vinyl acetate or alkyl acrylates improve adhesive qualities over polyethylene alone partly because of their polar nature. Alkyl acrylates however are known to provide more stable ethylene copolymers than vinyl acetate. The higher n-alkyl acrylates, which produce low glass transition temperature, also produce lower glass transition temperature ethylene copolymers. This contributes to `softness` of the adhesive, imparting greater adhesivity, particularly at lower temperatures. Highly polar acidic monomers such as acrylic and methacrylic acid are also known to be excellent comonomers for ethylene copolymer adhesives.
Reduction in crystallinity resulting from copolymerization also imparts adhesivity. In ethylene copolymers, both lower crystallinity and lower glass transition temperature result in `softer` more adhesive polymers at ambient and low temperatures. However there will also be a tendency to creep more. Such ethylene copolymers exemplify the statement above, that `soft` adhesives become weak at high temperature, and are less effective adhesives at higher temperatures.
Various means have been used to counter this tendency. One means is use of block copolymers or graft copolymers with a soft and a hard segment. The segments are blocks or grafts consisting of a moderate to long polymeric sequence. Another approach is to crosslink the adhesive after it has been applied as a hot melt, by such means as radiation or chemical post-reaction techniques. Such techniques have long been known to reduce creep and improve other properties, particularly in wire coating uses.
Post-reaction crosslinking or `curing` by radiation has many disadvantages with respect to equipment and other limitations. Post-reaction by purely chemical means is an alternative. This might involve any number of chemical reactions, but one of great convenience is `silane` crosslinking. Here, moisture is allowed to hydrolyse certain silane groups leading to crosslinking.
To achieve silane crosslinking in ethylene copolymers it is necessary to incorporate a silane group in the copolymer. Ethylene copolymers containing silane groups are known, and have been prepared by at least three techniques. One is to copolymerize ethylene with vinyl silanes. Another is to graft vinyl silanes onto polyethylene or direct ethylene copolymers using peroxides or other free-radical initiators. Moisture-curable vinyl silane grafted polyolefins suitable for crosslinked wire and cable coatings are described in U.S. Pat. No. 4,549,041 (Shingo). This reference also discusses the advantages and disadvantages of silane crosslinking. One of the disadvantages of free radical vinyl silane grafting is the control of silane level, and the ability to obtain adequate silane levels.
A third known method of incorporating silane functionality is by another type of graft reaction. In this method, a direct copolymer containing a suitable reactive group is reacted with a silane containing compound which also contains a complementary reactive group to that in the direct copolymer. The result is a graft copolymer containing a reactive silane group. Here the graft is a simple reactive unit, not to be confused with the above mentioned graft copolymer containing a polymeric unit grafted on, where the graft unit itself is designed to increase high temperature properties.
Preparation of silane functional ethylene copolymers by this third method is described in U.S. Pat. No. 3,408,420 (Wiggill). This reference describes reaction of a wide range of ethylene copolymers containing a hydroxyl, carboxylic or acid chloride group with silane compounds containing a complementary reactive group such as a hydroxyl, secondary amino or epoxy group. The compounds are described as being useful as coatings on metal or glass and paper, and in the formation of fibers and films. In all cases preparation involved a solution of the reactive silane in an organic solvent. In addition, high levels of reactive monomer were present in the direct copolymer, and this results in an unstable polymer.
There is a need for an ethylene based silane containing polymeric composition which can both be prepared in the melt and be sufficiently melt stable to be used in the melt as an adhesive, and which can subsequently be crosslinked at an adequate rate and in a controllable way.